1. Field of the Invention
The present invention relates to a marking line detection system and a marking line detection method.
2. Description of the Related Art
Japanese Patent Application Publication No. 2003-118521 (JP 2003-118521 A) is known as a detection system and a detection method for detecting a marking line on the road surface around a vehicle. Japanese Patent Application Publication No. 2003-118521 (JP 2003-118521 A) discloses a technology that initially sets the white line detection area in a captured image of the road in front of the vehicle as large as possible and, if the road parameters are updated later to reflect the actual road surface shape, sets the white line detection area as small as possible for increasing the processing speed while reducing the number of incorrect detections of non-white-line objects. This technology detects a distant white line by gradually shifting the white line detection area to fit the road surface shape while estimating the road parameters from the near side to the distant side of the vehicle. Japanese Patent Application Publication No. 2012-014398 (JP 2012-014398 A) discloses a technology that identifies a double white line, composed of a solid line and a broken line, on a line basis. This technology varies the identification method according to whether the area is near to the vehicle, where a double white line can be approximated almost as a straight line, or the area is distant from the vehicle where a double white line cannot be approximated almost as a double white line because the road, is a turning road. In this technology, because the solid line and the broken line of a double white line in a distant area are sometimes merged, the double white line, though provided for the vehicle, may be determined as a noise. Japanese Patent Application Publication No. 2010-060371 (JP 2010-060371 A) discloses a technology that calculates a histogram of edge intensities in the radiation direction from the vanishing point (determined in advance by the focus of the lens and the imaging surface) in the captured image to the vehicle side and, by identifying the direction in which the edge intensity becomes equal to or larger than the threshold, determines that there is a white line in that direction. Japanese Patent Application Publication No. 2011-170750 (JP 2011-170750 A) discloses a technology that records white line edges in a captured image for each frame but does not record white line edges for a frame when a pitch variation is generated in the vehicle. Japanese Patent Application Publication No. 2007-286724 (JP 2007-286724 A) discloses a technology that, when the vehicle enters an intersection, detects other vehicles that approach the intersection from the intersecting roads, based on the analysis result of the measuring points of each of the near area and the distant area that are set in the captured image of the road in front of the vehicle. In this technology, the density of the measuring points in the distant area is set higher than the density of the measuring points in the near area. Japanese Patent Application Publication No. 2001-101428 (JP 2001-101428 A) discloses a technology that changes the filter for extracting the edges of the preceding vehicle in the captured image of the road in front of the vehicle according to the distance between the vehicle and the preceding vehicle.
Incidentally, the technology for sequentially estimating the road surface shape while sequentially detecting the marking lines from the near side to the distant side has the following problems. For example, the road surface shape cannot sometimes be estimated correctly when the brightness value of the marking line is reduced by the shadow of a solid object such as a building, when the marking line is hidden by other vehicles or bicycles, or when the marking line is not detected continuously because the marking line is wavy. In such a case, the marking line cannot sometimes be detected during the estimation process because of insufficient information or because of the analysis of an incorrect area in the captured image. Therefore, there is room for increasing the detection accuracy of a distant marking line.